This invention relates to plotters of the type for drawing graphics on elongated webs of sheet material and wherein the sheet material is advanced over a support surface, the actual drawing process occurring on the portion of the material supported by the support surface and a finished drawing having a length significantly longer than the length of the support surface in the direction of sheet material advancement, and deal more specifically with a means for controlling the advancement or the sheet material in such a plotter.
Plotters of the type with which the invention is concerned are commonly used with paper being the sheet material on which the drawings are made by a writing implement, such as a pen or pencil, forming part of the plotter. In keeping with this, for convenience, in the following description and claims the sheet material has been referred to as "paper". Materials other than ordinary paper, such as webs of plastic or photographic film, may nevertheless sometimes be used as the sheet material. It is therefore intended that the word "paper", as used herein, be interpreted broadly to comprehend all types of sheet materials which may be used with the plotter in question.
In plotters of the type where an elongated web of paper is advanced over a support surface it is important that the paper advancement be accurately measured so that a portion of a drawing created on one section of the paper will match precisely with another portion drawn on an adjacent section of the paper. For example, the plotter may be operated in a section-by-section mode in which case the paper is held stationary relative to the support surface while a portion of a drawing is drawn on the paper overlying the support surface by a writing implement moved by the plotter in X and Y coordinate directions. When that portion of the drawing is completed the plotting process is suspended and the paper web is advanced over the support surface to bring to it a new paper section. Drawing is then resumed on the new section to create another portion of the finished drawing, and such alternation between drawing and paper advancement is repeated until the drawing is finished. This requires that the paper be accurately located at the finish of each advancement for proper registration of adjacent portions of the drawing. It also is important for efficiency of throughput that the paper advancements be accomplished in as short a time as possible.
In plotters of the aforementioned type, it is also generally necessary prior to beginning a drawing that the paper means used to sense movement of the paper in the direction of advancement be set to an initialized condition in order for it to properly indicate the position relative to a reference point fixed to the support surface of a given reference point on the paper, and this initializing process often involves the wasteage of a substantial amount of paper.
The general object of the invention is therefore to provide a paper advance control system for a plotter wherein a paper web is moved over a support surface with the control achieving accurate positioning of the paper relative to the support surface. In keeping with this object a more particular object is to provide such a paper advance control particularly suited for operation of the plotter in a section-by-section mode and capable of assuring precise positioning of the paper relative to the support surface at the end of each paper advancement.
A further object of the invention is to provide a paper advance control of the foregoing character whereby the paper advancements are accomplished in relatively short periods of time to maximize the throughput of the plotter. In keeping with this object, the paper advancement means includes a supply roll and a take-up roll for the paper with the take-up roll being driven by a motor to achieve the paper advancement and with the motor being so energized as to achieve a predetermined uniform speed of the paper over the support surface throughout the major portion of each advancement regardless of the diameter of the take-up roll.
Another object of the invention is to provide a paper advance control system of the above-identified character including a paper advancement sensor which may be set to an initialized position relative to the paper without the potential for significant paper wastage.
Other objects and advantages of the invention will be apparent from the following description of a preferred embodiment and from the accompanying drawings and claims.